Sulfonamide-induced reactions in desensitized patients with AIDS--the role of covalent protein haptenation by sulfamethoxazole

Immunogenicity of trimethoprim/sulfamethoxazole in a macaque model of HIV infection

BACKGROUND

Sulfonamide hypersensitivity has a high incidence in HIV infection and correlates with low CD4+ counts, but the mechanisms are not understood. The aims of this study were to determine whether trimethoprim/sulfamethoxazole (TMP/SMX) led to SMX adduct formation, immunogenicity, or signs of drug hypersensitivity in SIV-infected rhesus macaques, and whether differences in antioxidants, pro-inflammatory mediators, or SMX disposition were predictive of drug immunogenicity.

METHODS

Nine macaques chronically infected with SIVmac239 and 7 non-infected controls were studied. Baseline blood ascorbate, glutathione, IFN-γ, LPS, sCD14, and cytochrome b₅ reductase measurements were obtained, macaques were dosed with TMP/SMX (120mg/kg/day p.o. for 14days), and SMX metabolites, lymph node drug adducts, drug-responsive T cells, and anti-SMX antibodies were measured.

RESULTS

Four of 9 of SIV-positive (44%), and 3 of 7 SIV negative (43%) macaques had drug-responsive T cells or antibodies to SMX. Two macaques developed facial or truncal rash; these animals had the highest levels of lymph node drug adducts. Antioxidants, pro-inflammatory mediators, and SMX metabolites were not predictive of drug immunogenicity; however, the Mamu DRB1*0401/0406/0411 genotype was significantly over-represented in immune responders.

CONCLUSIONS

Unlike other animal models, macaques develop an immune response, and possible rash, in response to therapeutic dosages of TMP/SMX. Studying more animals with CD4+ counts <200cells/μl, along with moderately restricted ascorbate intake to match deficiencies seen in humans, may better model the risk of SMX hypersensitivity in HIV-infection. In addition, the role of Mamu-DRB1 genotype in modeling drug hypersensitivity in retroviral infection deserves further study.

Characterization of Peroxidases Expressed in Human Antigen Presenting Cells and Analysis of the Covalent Binding of Nitroso Sulfamethoxazole to Myeloperoxidase

Drug hypersensitivity remains a major concern, as it causes high morbidity and mortality. Understanding the mechanistic basis of drug hypersensitivity is complicated by the multiple risk factors implicated. This study utilized sulfamethoxazole (SMX) as a model drug to (1) relate SMX metabolism in antigen presenting cells (APCs) to the activation of T-cells and (2) characterize covalent adducts of SMX and myeloperoxidase, which might represent antigenic determinants for T-cells. The SMX metabolite nitroso-SMX (SMX-NO) was found to bind irreversibly to APCs. Time- and concentration-dependent drug-protein adducts were also detected when APCs were cultured with SMX. Metabolic activation of SMX was significantly reduced by the oxygenase/peroxidase inhibitor methimazole. Similarly, SMX-NO-specific T-cells were activated by APCs pulsed with SMX, and the response was inhibited by pretreatment with methimazole or glutaraldehyde, which blocks antigen processing. Western blotting, real-time polymerase chain reaction (RT-PCR), and mass spectrometry analyses suggested the presence of low concentrations of myeloperoxidase in APCs. RT-PCR revealed mRNA expression for flavin-containing monooxygenases (FMO1-5), thyroid peroxidase, and lactoperoxidase, but the corresponding proteins were not detected. Mass spectrometric characterization of SMX-NO-modified myeloperoxidase revealed the formation of N-hydroxysulfinamide adducts on Cys309 and Cys398. These data show that SMX's metabolism in APCs generates antigenic determinants for T-cells. Peptides derived from SMX-NO-modified myeloperoxidase may represent one form of functional antigen.

Successful outpatient graded administration of trimethoprim-sulfamethoxazole in patients without HIV and with a history of sulfonamide adverse drug reaction

BACKGROUND

The outcomes of trimethoprim-sulfamethoxazole (TMP-SMX) desensitization have been widely reported in the HIV literature but less so in the non-HIV literature.

OBJECTIVE

To evaluate the safety and efficacy of graded administration of TMP-SMX in patients without HIV and with a history of TMP-SMX adverse drug reaction (ADR).

METHODS

A retrospective chart review, 2004-2012, of all the patients without HIV seen in the Division of Allergic Diseases and with a history of TMP-SMX ADR who underwent outpatient graded administration of TMP-SMX was conducted. The medical record was reviewed for age, sex, details of the initial ADR to TMP-SMX, an indication for TMP-SMX administration, and outcome. Patients also were contacted by telephone, and medical records were reviewed to determine long-term outcomes.

RESULTS

Seventy-two patients (46 women [64%]; mean [SD] age, 57.7 ± 13.89 years]) were included. The most common patient-reported reactions to TMP-SMX were rash 39 (54%), and hives 9 (13%). TMP-SMX administration was needed for the following indications: prophylaxis (62 [86%]) and treatment of infection (10 [14%]). Forty-three of the patients (60%) underwent a 1-day TMP-SMX administration protocol. Thirty-five of the 43 (81%) underwent a 6-step (90 minutes to 6 hours) protocol and 7 of the 43 (16%) underwent a novel 14-step TMP-SMX protocol. Twenty-nine (40%) underwent a >1-day TMP-SMX administration protocol. Our overall success rate was 90% (mean duration of 11 months). Ninety-eight percent of the patients successfully completed a 1-day graded administration protocol, and 76% successfully completed a >1-day protocol. TMP-SMX was stopped in 8 patients because of the ADR.

CONCLUSION

We report the largest case series of successful outpatient graded administration of TMP-SMX with both 1-day and >1-day protocols, which have shown to be safe and well tolerated in patients without HIV and with a history of sulfonamide ADR.

Covalent binding of nitroso-sulfonamides to glutathione S-transferase in guinea pigs with delayed type hypersensitivity

Drug induced allergies are believed to be induced by conjugates consisting of biological macromolecules and active metabolites. The present study investigated whether guinea pig glutathione S-transferase (gpGST), a protein that binds with sulfanilamide (SA) and sulfamethoxazole (SMX), could be detected in the liver cytosol fraction of guinea pigs that intraperitoneally received SA or SMX, and whether gpGST is a carrier protein. We synthesized three nitroso compounds, i.e., 4-nitroso-sulfanilamide (SA-NO), 4-nitrososulfamethoxazole (SMX-NO) and fluorescent-labeled nitroso compound (DNSBA-NO), and examined binding quantities of nitroso compounds to gpGST purified from untreated female guinea pigs. Furthermore, the concentrations of IgG in serum antibody for nitroso compounds were estimated using ELISA. When guinea pigs were sensitized using the three nitroso compounds, the dose dependent skin reactions were confirmed with each compound. In addition, sensitized guinea pigs using each nitroso compound showed positive skin reactions at an elicitation test performed using gpGST alone. The results confirmed synthesis of antibody against gpGST due to hapten sensitization. Therefore, when a nitroso compound binds with gpGST in the body of guinea pigs, nitroso-gpGST acts as a neoantigen, which induces synthesis of autoantibody. Thus, gpGST appears to be one of the carrier proteins that induce sulfa drug-induced allergies. Immunization of guinea pigs with active metabolite of drugs may give information for predicting the occurrence of delayed type hypersensitivity in human.

Clinical applications of drug desensitization in the Asia-Pacific region

Drug desensitization is the induction, within hours to days, of a temporary state of tolerance to a drug which the patient has developed a hypersensitivity reaction to. It may be used for IgE and non-IgE mediated allergic reactions, and certain non-allergic reactions. The indication for desensitization is where no alternative medications are available for the treatment of that condition, and where the benefits of desensitization outweigh the risks. Desensitization is a therapeutic modality for drug allergy (similar to allergen specific immunotherapy for allergic rhinitis and insect venom anaphylaxis). In contrast, the drug provocation test is a diagnostic modality used to confirm or refute the diagnosis of drug allergy. This review discusses the clinical applications of desensitization for the treatment of common infectious, metabolic and cardiovascular diseases, and oncological conditions in the Asia-Pacific region.

Multiple adduction reactions of nitroso sulfamethoxazole with cysteinyl residues of peptides and proteins: implications for hapten formation

Sulfamethoxazole (SMX) induces immunoallergic reactions that are thought to be a result of intracellular protein haptenation by its nitroso metabolite (SMX-NO mass, 267 amu). SMX-NO reacts with protein thiols in vitro, but the conjugates have not been defined chemically. The reactions of SMX-NO with glutathione (GSH), a synthetic peptide (DS3), and two model proteins, human GSH S-transferase pi (GSTP) and serum albumin (HSA), were investigated by mass spectrometry. SMX-NO formed a semimercaptal (N-hydroxysulfenamide) conjugate with GSH that rearranged rapidly (1-5 min) to a sulfinamide. Reaction of SMX-NO with DS3 also yielded a sulfinamide adduct (mass increment, 267 amu) on the cysteine residue. GSTP was exclusively modified at the reactive Cys47 by SMX-NO and exhibited mass increments of 267, 283, and 299 amu, indicative of sulfinamide, N-hydroxysulfinamide, and N-hydroxysulfonamide adducts, respectively. HSA was modified at Cys34, forming only the N-hydroxysulfinamide adduct. HSA modification by SMX-NO under these conditions was confirmed with ELISA and immunoblotting with an antisulfonamide antibody. It is proposed that cysteine-linked N-hydroxysulfinamide and N-hydroxysulfonamide adducts of SMX are formed via the reaction of SMX-NO with cysteinyl sulfoxy acids. Evidence for a multistep assembly of model sulfonamide epitopes on GSH and polypeptides via hydrolyzable intermediates is also presented. In summary, novel, complex, and metastable haptenic structures have been identified on proteins exposed in vitro to the nitroso metabolite of SMX.

Antigenicity of sulfanilamide and its metabolites using fluorescent-labelled compounds

In order to clarify the onset mechanisms of drug-induced allergies, three fluorescent-labelled compounds were synthesized by subjecting sulfanilamide (SA), a base compound for sulfonamides, and its active metabolites, i.e. sulfanilamide hydroxylamine and sulfanilamide nitroso, to dansylation using dansylchloride. In other words, 5-dimethylamino-N-(4-aminobenzyl)-naphthalenesulfonamide (DNS-4ABA), 5-dimethylamino-N-(4-hydroxylaminobenzyl)-1-naphthalenesulfonamide (DNS-4HABA) and 5-dimethylamino-N-(4-nitrosobenzyl)-1-naphthalenesulfonamide (DNS-4NSBA) were synthesized as model haptens. When analysed by HPLC, a conjugate of DNS-4HABA and glutathione (GSH) with nucleophilic amino acids had two peaks (P-1 and P-2). FAB-MS and 1H-NMR revealed that the DNS-4HABA-GSH conjugate consisted of sulphinamide and semimercaptal. The reactivity of GSH to DNS-4ABA, DNS-4HABA and DNS-4NSBA was quantified by HPLC using an oxidization system (horseradish peroxidase/H2O2). The results show that production of DNS-4NSBA-GSH-conjugate was four to eight times higher than that of DNS-4HABA-GSH conjugate, but that DNS-4ABA did not bind with GSH. Skin reactions were assessed using guinea pigs, and strong delayed erythema was seen with DNS-4NSBA, which bound most strongly with GSH, whereas weak delayed erythema was seen with DNS-4ABA, which did not bind with GSH. This suggests a correlation between GSH conjugate production and skin reactions. DNS-4HABA enzymatically bound with proteins in rat and guinea pig liver cytosol and microsomal fractions. The proteins that bound to DNS-4HABA were purified by HPLC and then subjected to N-terminal amino acid analysis. Ubiquitin (10 kDa) and fatty acid binding protein (30 kDa) were detected in the rat liver cytosol fraction; retinol-dehydrogenase (35 kDa) in the rat microsomal fraction; and glutathione-S-transferase B (mmu) (25 kDa) in the guinea pig liver cytosol fraction. When DNS-4HABA or DNS-4NSBA binds to proteins that play important roles in the body, unexpected adverse reactions may occur. Furthermore, by utilizing our technique using model compounds, it may be possible to identify the carrier proteins of various compounds, including pharmaceutical agents.

Sulfonamide allergy and cross-reactivity

Concerns about cross-allergenicity between sulfonamide antibiotics and nonantibiotic sulfonamide-containing drugs continue to complicate pharmacotherapy. Several elegant investigations have demonstrated unequivocal lack of interaction between the sulfonamide group and either cellular or humoral immunity. The immunologic determinant of type I immunologic responses to sulfonamide antibiotics is the N1 heterocyclic ring, and nonantibiotic sulfonamides lack this structural feature. Many non-type I hypersensitivity responses to sulfonamide antibiotics are attributable to reactive metabolites that cause either direct cytotoxicity or humoral or cellular responses. Metabolite formation is stereospecific to the N4 amino nitrogen of the sulfonamide antibiotics, a structure not found on any nonantibiotic sulfonamide drugs. Cellular immune responses to sulfonamide antibiotics are responsible for many non-immunoglobulin E-mediated dermatologic reactions; however, the stereospecificity of T-cell response renders cross-reactivity between sulfonamide antibiotics and nonantibiotics highly unlikely. Apparent cross-reactivity responses to sulfonamide-containing drugs likely represent multiple concurrent, rather than linked, drug hypersensitivities.

Increased adverse drug reactions to antimicrobials and anticonvulsants in patients with HIV infection

OBJECTIVE

To review the incidence, signs, symptoms, and mechanisms of adverse drug reactions (ADRs) to sulfonamides, anticonvulsants, and antimycobacterial medications among people with HIV.

DATA SOURCES

Searches of MEDLINE/PubMed (1980-November 2005) and National Library of Medicine Meeting Abstracts (1989-November 2005), as well as hand searches of journals and abstracts, were conducted to identify primary literature. Reference lists were reviewed to identify additional relevant reports.

STUDY SELECTION AND DATA EXTRACTION

Relevant articles and abstracts, particularly of in vitro experiments and clinical studies, were compiled and reviewed.

DATA SYNTHESIS

ADRs, especially in HIV-infected patients, are a cause for concern. Sulfonamides, anticonvulsants, and antimycobacterial drugs are commonly used to prevent and treat complications of HIV, including seizures and opportunistic infections. Patients with HIV have a much greater rate of ADRs to these drug classes, including severe and life-threatening hypersensitivity reactions. Several mechanisms of these ADRs have been postulated. Sulfamethoxazole and anticonvulsant hypersensitivity may involve the increased formation and decreased detoxification of reactive metabolites. The mechanisms for the marked increase in hypersensitivity ADRs to antimycobacterial drugs may be related to an altered immune profile in patients infected with both tuberculosis and HIV.

CONCLUSIONS

ADRs to antimicrobial and anticonvulsant therapy cause markedly increased morbidity and mortality in HIV-positive patients. Further research involving the interaction between HIV and the increased ADRs to these drugs is required.

Association of drug-serum protein adducts and anti-drug antibodies in dogs with sulphonamide hypersensitivity: A naturally occurring model of idiosyncratic drug toxicity

BACKGROUND

Sulphonamide antimicrobials, such as sulphamethoxazole (SMX), provide effective infection prophylaxis in immunocompromised patients, but can lead to drug hypersensitivity (HS) reactions. These reactions also occur in dogs, with a similar time course and clinical presentation as seen in humans.

OBJECTIVES

Drug-serum adducts and anti-drug antibodies have been identified in sulphonamide HS humans. The aim of this study was to determine whether similar markers were present in dogs with sulphonamide HS.

METHODS

Thirty-four privately owned sulphonamide HS dogs, 10 sulphonamide-'tolerant' dogs, 18 sulphonamide-naïve dogs, and four dogs experimentally dosed with SMX and the oxidative metabolite SMX-nitroso, were tested for drug-serum adducts by immunoblotting, and anti-drug antibodies by ELISA.

RESULTS

Sulphonamide-serum adducts were found in 10/20 HS dogs tested (50%), but in no tolerant dogs. Anti-sulphonamide IgG antibodies were detected in 17/34 HS dogs (50%), but in only one tolerant dog; antibody absorbance values were significantly higher in HS dogs. There was a significant association between the presence of sulphonamide-serum adducts and anti-sulphonamide antibodies (P = 0.009). Anti-drug antibodies were also found in dogs experimentally dosed with SMX-nitroso followed by SMX, but not in a dog dosed with drug vehicle, followed by SMX.

CONCLUSION

Similar humoral markers are present in dogs and humans with sulphonamide HS, supporting the use of dogs as a naturally occurring model for this syndrome in humans. These data suggest the potential use of drug-serum adducts and anti-drug antibodies as markers for sulphonamide HS. Preliminary data indicate that anti-sulphonamide antibodies may be triggered by the SMX-nitroso metabolite, not by the parent drug, in dogs.

Intravenous Linezolid Administered Orally: A Novel Desensitization Strategy

A 41-year-old woman with a history of myasthenia gravis was admitted to a local hospital because of severe muscle weakness, ptosis, shortness of breath, nausea and vomiting, and fever. Blood cultures revealed Enterococcus faecium resistant to several antimicrobial agents. The organism had minimum inhibitory concentrations above 16 microg/ml for vancomycin and above 2 microg/ml for quinupristin-dalfopristin. In the absence of therapeutic alternatives, treatment with linezolid was required (minimum inhibitory concentration 1.5 microg/ml). The first dose of linezolid resulted in a hypersensitivity reaction consistent with an immunoglobulin E-mediated response requiring medical intervention. Because of a lack of intravenous access and because of limited availability of the oral suspension from the manufacturer, a desensitization protocol was implemented in which the intravenous formulation of linezolid was given orally. The patient was successfully desensitized by using an escalating, 14-dose procedure. We believe this is the first case in the English language literature to describe successful desensitization with the oral administration of intravenous linezolid in a patient with E. faecium bacteremia who was allergic to oxazolidinone.

Characterization of the Formation and Localization of Sulfamethoxazole and Dapsone-Associated Drug-Protein Adducts in Human Epidermal Keratinocytes

Sulfonamide- and sulfone-induced hypersensitivity reactions are thought to be mediated through bioactivation of parent drug molecule(s) to their respective reactive metabolite(s). Recent studies have demonstrated that keratinocytes can bioactivate sulfonamides and sulfones. Using enzyme-linked immunosorbent assay and hapten-specific rabbit antisera developed in our laboratory, we found that incubation of either normal human epidermal keratinocytes (NHEKs) or an immortalized human keratinocyte cell line (HaCaT) with sulfamethoxazole (SMX) or dapsone (DDS) resulted in the formation of drug/metabolite protein adducts. The formation of these adducts with SMX was increased in the presence of ascorbic acid, whereas N-acetylcysteine decreased adduct formation with both SMX and DDS. Adduct formation was confirmed using confocal microscopy when NHEKs were incubated with SMX, DDS, or their respective arylhydroxylamine metabolites. Cellular distribution of adducts was compared in permeable versus nonpermeable NHEKs. Exposure to SMX, DDS, or dapsone hydroxylamine resulted in the formation of intracellular adducts, whereas SMX hydroxylamine also resulted in the presence of adducts on the cell surface. In summary, our work shows that keratinocytes can bioactivate SMX/DDS to form drug-protein adducts, which may be acquired by antigen-presenting cells upon keratinocyte cell death, evoking an immune response. In addition, keratinocytes may themselves present antigen to hapten-specific cytotoxic T lymphocytes. Furthermore, our results also suggest that different sulfonamides/sulfones may have different protein targets for in situ haptenation in keratinocytes.

Evaluation of the clinical, immunologic, and biochemical effects of nitroso sulfamethoxazole administration to dogs: a pilot study

Sulfonamide antimicrobials such as sulfamethoxazole (SMX) have been associated in humans with hypersensitivity reactions, to include fever, skin eruptions, hepatotoxicity, and blood dyscrasias. These reactions also occur in dogs, the only non-human species known to develop a similar spectrum of sulfonamide hypersensitivity. Sulfonamide hypersensitivity is not well understood, but has been hypothesized to be due to the generation of the reactive oxidative metabolite, nitroso sulfamethoxazole (SMX-NO). SMX-NO, unlike the parent sulfonamide, is cytotoxic in vitro, haptenizes tissue proteins, and is immunogenic in rodents. The purpose of this pilot study was to determine whether SMX-NO, when administered to dogs, would lead to drug-tissue adducts, anti-drug antibodies, antioxidant depletion, or clinical evidence of drug hypersensitivity. Four dogs were randomized to one of four treatments: SMX-NO 1 mg/kg; SMX-NO 3 mg/kg; SMX-NO 10 mg/kg; or vehicle control. Dosing was by the intraperitoneal route, once daily for four consecutive days per week, for 2 weeks total, followed by a third week of observation. Following this, all dogs were challenged with trimethoprim-sulfamethoxazole, 25 mg/kg for 12 h for 2 weeks. No dog developed clinical or biochemical evidence of drug hypersensitivity. Plasma cysteine and leukocyte reduced glutathione were not depleted during dosing; however, ascorbate was significantly depleted by week 2 following SMX-NO at 10 mg/kg. Anti-SMX antibodies (IgG or IgM by ELISA) were not detected in any dogs at any time points. SMX-hemoglobin adducts were detected in the spleen in SMX-NO dosed dogs; however, these adducts were not accompanied by an immunologic or systemic response. The results of this pilot study indicate that SMX-NO dosing in dogs, using a dosing protocol shown to be immunogenic in other species, produces modest ascorbate depletion and hemoglobin adduct formation, but is insufficient to produce an immunologic response or a clinical syndrome of sulfonamide hypersensitivity in this susceptible species.

Idiosyncratic toxicity associated with potentiated sulfonamides in the dog

Idiosyncratic toxicity to potentiated sulfonamides occurs in both humans and dogs, with considerable clinical similarities. The syndrome in dogs can consist of fever, arthropathy, blood dyscrasias (neutropenia, thrombocytopenia, or hemolytic anemia), hepatopathy consisting of cholestasis or necrosis, skin eruptions, uveitis, or keratoconjunctivitis sicca. Other manifestations seen less commonly include protein-losing nephropathy, meningitis, pancreatitis, pneumonitis, or facial nerve palsy. The pathogenesis of these reactions is not completely understood, but may be due to a T-cell-mediated response to proteins haptenated by oxidative sulfonamide metabolites. Our laboratory is working on tests to characterize dogs with possible idiosyncratic sulfonamide reactions, to include ELISA for anti-drug antibodies, immunoblotting for antibodies directed against liver proteins, flow cytometry for drug-dependent anti-platelet antibodies, and in vitro cytotoxicity assays. The management of idiosyncratic sulfonamide toxicity involves client education to identify clinical signs early and allow rapid drug discontinuation, supportive care to include possibly ascorbate and glutathione precursors, and avoidance of subsequent re-exposure. It is important to realize that only antimicrobial sulfonamides, such as sulfamethoxazole, sulfadiazine, and sulfadimethoxine, share this clinical syndrome. There is no evidence for cross-reactivity with drugs that have different underlying structures but share a sulfonamide moiety, such as acetazolamide, furosemide, glipizide, or hydrochlorthiazide.

10. Drug allergy

Adverse drug reactions are common, but only 6% to 10% are immunologically mediated. Unlike most adverse drug reactions, allergic drug reactions are unpredictable. Whereas some drug-induced allergic reactions may be easily classified into one of the four Gell and Coombs hypersensitivity categories, many others that appear to have an immunologic component cannot be classified because of our lack of mechanistic information. Theoretically, any drug can induce an immune response. However, some drugs are more likely to elicit clinically relevant immune responses than are others. Drugs in this category include antimicrobial drugs, anticonvulsants, chemotherapeutic agents, heparin, insulin, protamine, and biologic response modifiers. After a drug-disease connection is established, it must be determined whether the reaction was immunologically mediated. Subsequently, confirmatory tests, if available, should be used to determine the allergic status of the patient. If these tests are not available, a graded challenge or desensitization may be considered, depending on the type of clinical reaction previously demonstrated and the need for drug readministration. Education of the patient and primary care physician is an important component of patient management.

The danger hypothesis--potential role in idiosyncratic drug reactions

Idiosyncratic or type B reactions are characterised by their unpredictability and lack of simple dose-dependency. They occur in a small proportion of patients, and usually the predisposing factors are unknown. A proportion of, but not all, idiosyncratic reactions are immune-mediated. Our understanding of immune-mediated reactions is based on the hapten hypothesis, which requires drug bioactivation, covalent binding to proteins, followed by uptake, antigen processing and a polyclonal immune response. The recently proposed 'danger hypothesis' can be considered to be additive to the hapten hypothesis. The hypothesis states that the immune system only responds when it detects danger. If no danger is detected, tolerance results. Thus, stimulation of an immune response to a drug-protein conjugate (signal 1) requires the presence of co-stimulatory signals and cytokines (signals 2 and 3), which propagate and determine the type of immune response. The nature of the danger signal is poorly defined, and has been proposed to include different forms of stress including chemical, physical and viral. Indeed, there are several examples where the frequency of drug hypersensitivity is increased in the presence of a viral infection, most notably in HIV disease. Nevertheless, this clinical evidence has to be regarded as being circumstantial and more direct experimental evidence is required to understand the role of 'danger' in the overall pathogenesis of drug hypersensitivity reactions.

Novel non-labile covalent binding of sulfamethoxazole reactive metabolites to cultured human lymphoid cells

Sulfamethoxazole (SMX) causes rare hypersensitivity syndrome reactions characterized by fever and multi-organ toxicity. Covalent binding of SMX reactive metabolites to cellular proteins has been demonstrated but the link between cytotoxicity and targets of covalent binding has not been explored. We therefore investigated the relationship between covalent binding of the reactive SMX-hydroxylamine (SMX-HA) metabolite, and its cytotoxicity to a hystiocytic lymphoma (U937) cell line. Incubation of U937 cells with 0-1 mM SMX-HA for 3 h resulted in dose-dependent cytotoxicity, as assessed by tetrazolium dye conversion at 24 h. SMX-HA caused dose-dependent covalent binding to cellular proteins as assessed by immunoblotting with SMX antisera at 3 and 24 h. Covalent binding was predominantly to proteins of approximately 45, 59 and 75 kDa, but other targets were also observed. The relative extent of binding to proteins was significantly different from the relative cytotoxicity at 24 h. Further, cells surviving at 24 h also had extensive covalent binding. Covalent binding was observed under reducing (beta-mercaptoethanol) and non-reducing conditions to plasma membrane and microsomal but not cytosolic proteins. This non-labile covalent binding has not been previously reported. These observations suggest that extensive covalent binding does not necessarily lead to cell death, allowing the accumulation of potentially immunogenic drug-protein conjugates. These observations in whole cells may be relevant to the immunopathogenesis of SMX hypersensitivity syndrome reactions.

Allergic adverse reactions to sulfonamides

Antimicrobial sulfonamides were the first antimicrobial agents used effectively to treat infectious diseases. However, because they may cause severe adverse drug reactions (ADRs) and because more effective agents have since been developed, sulfonamides now are used for only a few indications in specific groups, such as AIDS patients. Skin reactions, from benign rash to potentially lethal toxidermias, are the most frequent ADRs to sulfonamides. Other major ADRs include acute liver injury, pulmonary reactions, and blood dyscrasias. Although the mechanisms involved have not been fully elucidated, reactive metabolites appear to play a pivotal role. The hydroxylamine and nitroso metabolites of sulfamethoxazole, the most frequently used sulfonamide today, can bind covalently to proteins because of their chemical reactivity, resulting in the induction of specific adverse immune responses. Therefore, changes in the activity of metabolic and detoxification pathways are associated with a greater risk for developing allergic reactions to sulfonamides. Allergies to sulfonamides, particularly sulfamethoxazole (often used in combination with trimethoprim as co-trimoxazole), are more frequent in AIDS patients, but the reason for this increased risk is not fully understood. No valid tools are available to predict which patients have a greater risk for developing allergies to sulfonamides. Diagnosis is essential to avoid a possible evolution toward severe reactions and readministration of the offending drug. In patients who absolutely require further treatment, successful desensitization may be achieved.

Drug metabolism, danger signals, and drug-induced hypersensitivity

One of the most difficult challenges for the practicing allergist/immunologist today is that of evaluating and managing patients who present with histories of drug-induced reactions. Adverse drug reactions are heterogeneous, and a single drug can often cause a multitude of reactions. Because the mechanisms responsible for many of these reactions are not known, they can be, and often are, difficult to classify. Moreover, for those that have features consistent with immune-mediated mechanisms, our diagnostic tools remain limited, because little is known about the relevant immunogenic determinants of most drugs. Despite these challenges, management approaches must be devised for patients who present with histories of drug-induced disease. Simply telling such a patient to avoid all drugs that have been associated with previous adverse events leaves both the patient and the referring physician frustrated. The initial part of this review focuses on exciting current research that is furthering our understanding of the mechanisms responsible for drug-induced reactions. Because it will take time to translate this new information into clinical practice, the latter part of the review focuses on ways to evaluate and manage patients who present with drug-induced reactions using the tools and the knowledge that are currently available.

Antigenicity and immunogenicity of sulphamethoxazole: demonstration of metabolism-dependent haptenation and T-cell proliferation in vivo

Sulphamethoxazole has been associated with the occurrence of hypersensitivity reactions. There is controversy as to whether the immune response is metabolism-dependent or -independent. We have therefore investigated the site of antigen formation and the nature of the drug signal presented to the immune system in vivo. Male Wistar rats were dosed with sulphamethoxazole, sulphamethoxazole hydroxylamine or nitroso sulphamethoxazole. Antigen formation on cell surfaces was determined by flow cytometry using a specific anti-sulphamethoxazole antibody. Immunogenicity was determined by assessment of ex vivo T-cell proliferation. Administration of nitroso sulphamethoxazole, but not sulphamethoxazole or sulphamethoxazole hydroxylamine, resulted in antigen formation on the surface of lymphocytes, splenocytes and epidermal keratinocytes, and a strong proliferative response of splenocytes on re-stimulation with nitroso sulphamethoxazole. Rats dosed with sulphamethoxazole or sulphamethoxazole hydroxylamine did not respond to any of the test compounds. CD4+ or CD8+ depleted cells responded equally to nitroso sulphamethoxazole. The proliferative response to nitroso sulphamethoxazole was seen even after pulsing for only 5 min, and was not inhibited by glutathione. Responding cells produced IFN-gamma, but not IL-4. Haptenation of cells by sulphamethoxazole hydroxylamine was seen after depletion of glutathione by pre-treating the rats with diethyl maleate. Splenocytes from the glutathione-depleted sulphamethoxazole hydroxylamine-treated rats responded weakly to nitroso sulphamethoxazole, but not to sulphamethoxazole or sulphamethoxazole hydroxylamine. Dosing of rats with sulphamethoxazole produced a cellular response to nitroso sulphamethoxazole (but not to sulphamethoxazole or its hydroxylamine) when the animals were primed with complete Freund's adjuvant. These studies demonstrate the antigenicity of nitroso sulphamethoxazole in vivo and provide evidence for the role of drug metabolism and cell surface haptenation in the induction of a cellular immune response in the rat.

Plasma cysteine deficiency and decreased reduction of nitrososulfamethoxazole with HIV infection

The aim of these studies was to determine whether HIV-infected patients have a plasma thiol deficiency and whether this is associated with decreased detoxification of the toxic metabolites of sulfamethoxazole. Reduced, oxidized, protein-bound, and total thiol levels were measured in 33 HIV-positive patients and 33 control subjects by an HPLC method utilizing the fluorescent probe bromobimane. The reduction of sulfamethoxazole hydroxylamine and nitrososulfamethoxazole by plasma and the plasma redox balance in the presence of nitrososulphamethoxazole were also determined by HPLC. Reduced plasma cysteine was significantly (p<0.0001) lower in HIV-positive patients (13.0+/-3.0 microM) when compared with control subjects (16.9+/-3.0 microM). Although there was no difference in oxidized, protein-bound, and total cysteine, the thiol/disulfide ratios were lower in HIV-positive patients. Reduced homocysteine was elevated in patients. Plasma from HIV-positive patients was less able to detoxify nitrososulfamethoxazole than control plasma. These findings show that the disturbance in redox balance in HIV-positive patients may alter metabolic detoxification capacity, and thereby predispose to sulfamethoxazole hypersensitivity.

Clinical assessment of drug-induced disease

Physicians are often confronted with patients who state that they are "allergic" to a drug or drugs. Knowing which medications can be prescribed safely is therefore difficult, and care of such patients frustrating. The goal of this review is to help physicians develop management plans for patients who present with drug-induced diseases. It provides information that allows physicians to differentiate between reactions that are truly allergic in nature and those that are not immunologically mediated. Relevant information on medical history, physical findings, and laboratory tests that may be helpful in the assessment are discussed, and guidance is provided on when and if a drug may be safely readministered. Unfortunately, however, until we are able to better understand the mechanisms responsible for drug-induced reactions, our management tools will remain limited.

Understanding drug allergies

At this time, the incidence of adverse drug reactions can only be estimated because the intensive monitoring and documenting that is required to make this determination does not exist at most hospitals and clinics. Despite these limitations, a meta-analysis of prospective studies has estimated the incidence of serious adverse drug reactions in hospitalized patients to be 6.7% and the incidence of fatal adverse drug reactions to be 0.32%. When evaluating and managing the condition of a patient who has experienced an adverse drug reaction, the physician first obtains an accurate history and performs a careful physical examination to determine whether the reaction was immunologic in nature. Drug reactions that are immunologically mediated (1) require a period of sensitization, (2) occur in a small proportion of the population, (3) are elicited at drug doses far below the therapeutic range, and (4) subside after drug discontinuation in most instances. All possible culprit drugs should be identified, with dosages and dates of administration and discontinuation, and the patient should be asked about any previous drug exposure history. Although immunodiagnostic tests for allergic drug reactions are limited, several tests do exist and may be useful in the identification of drug-specific antibodies, drug-specific T lymphocytes, or mediators from activated cells. If the reaction was not consistent with an IgE-mediated event and if it did not involve serious organ damage, cautious rechallenge may be considered. For those reactions that appear to be IgE-mediated and for which there is no reliable skin test reagent, drug desensitization may be performed by allergists who are trained in this procedure.

Recognising antibacterial hypersensitivity in children

Adverse reactions to antibacterial agents are not uncommon in children. They are classified as 'immediate' or 'nonimmediate' according to the time interval between drug administration and onset. Immediate reactions occur within 1 hour and are manifested by urticaria and/or angioedema, bronchospasm and anaphylactic shock; immunological reactions are mediated by IgE antibodies. The main nonimmediate reactions (occuring after more than 1 hour) are maculopapular rash, urticaria and serum sickness; T lymphocytes may participate in maculopapular rash. Clinical assessment of such reactions is complex. The patient's history is fundamental; the allergological examination includes in vivo and in vitro tests selected on the basis of the clinical features and the phase of reaction. In the late phase, prick and intradermal tests are sensitive in evaluating beta-lactam allergy. Together with delayed-reading intradermal testing, patch testing seems to be useful in diagnosing maculopapular reactions to systemically administered aminopenicillins. Determination of specific IgE levels is the most common in vitro method for diagnosing immediate reactions. In the acute phase, serum tryptase and urinary N-methylhistamine assays are reliable in diagnosing type I pathogenic mechanisms in immediate reactions. Unfortunately, there are few in vitro tests for evaluating other reactions, and most are not fully validated. In selected cases, provocation tests should be performed.

Cytotoxicity of sulfonamide reactive metabolites: apoptosis and selective toxicity of CD8(+) cells by the hydroxylamine of sulfamethoxazole

Treatment with sulfonamide antibiotics in HIV-infected patients is associated with a high incidence (> 40%) of adverse drug events, including severe hypersensitivity reactions. Sulfonamide reactive metabolites have been implicated in the pathogenesis of these adverse reactions. Sulfamethoxazole hydroxylamine (SMX-HA) induces lymphocyte toxicity and suppression of proliferation in vitro; the mechanism(s) of these immunomodulatory effects remain unknown. We investigated the cytotoxicity of SMX-HA via apoptosis on human peripheral blood mononuclear cells and purified cell subpopulations in vitro. CD19(+), CD4(+), and CD8(+) cells were isolated from human peripheral blood by positive selection of cell surface molecules by magnetic bead separation. SMX-HA induced significant CD8(+) cell death (67 +/- 7%) at 100 microM SMX-HA, with only minimal CD4(+) cell death (8 +/- 4%). No significant subpopulation toxicity was shown when incubated with parent drug (SMX). Flow cytometry measuring phosphatidylserine externalization 24 h after treatment with 100 microM and 400 microM SMX-HA revealed 14.1 +/- 0.7% and 25. 6 +/- 4.2% annexin-positive cells, respectively, compared to 3.7 +/- 1.2% in control PBMCs treated with 400 microM SMX. Internucleosomal DNA fragmentation was observed in quiescent and stimulated PBMCs 48 h after incubation with SMX-HA. Our data show that CD8(+) cells are highly susceptible to the toxic effects of SMX-HA through enhanced cell death by apoptosis.

Drug allergies

Allergic drug reactions are a significant cause of morbidity and mortality. Because it is difficult to identify the culprit drug and the underlying pathophysiologic mechanisms involved in these reactions, a systematic approach should be adopted in the evaluation of drug-allergic patients. Initially, the type of reaction should be determined. It should be realized that not all adverse reactions are allergic in nature. Allergic drug reactions comprise only a small category of adverse reactions in general. Therefore, the physician must determine if the reaction demonstrates features common to immunologic reaction. Subsequently, a detailed history should be obtained and a physical should be performed. Important information includes medication usage, previous drug exposure, current illness, family history of drug allergy and personal history of drug allergy. In managing the drug-allergic patient, the physician may choose to: select an alternative, non-cross-reacting drug if future therapy is needed; premedicate prior to future drug exposure if such regimens have been shown to be effective; or consult an allergist regarding the potential graded challenge or desensitization.